Spark plugs used in internal combustion engines are constantly subjected to environments having extreme temperatures and other potentially damaging elements. In order to combat such an aggressive environment and to protect the center electrode, certain precious metals are oftentimes employed at the firing end of the center electrode. These metals include platinum (Pt), iridium (Ir), and other noble metals exhibiting resistance to electrical erosion and chemical corrosion. It is known in the art to utilize these metals in the form of an insert; that is, a small, solid precious metal piece added to the lowermost tip of the center electrode. This provides a durable sparking surface without having to construct the entire electrode out of the precious metal, which could be economically impractical. Although the use of a precious metal insert may improve the consumption resistance (operational lifetime) and performance characteristics (ignitability) of the spark plug, its use may introduce new design and manufacturing difficulties. One of those difficulties involves the attachment of the insert to the firing tip in a manner capable of withstanding the extreme forces exerted upon it during use.
One method of attaching a precious metal insert to the firing tip of a center electrode is shown in U.S. Publication No. 2001/0030495 A1 issued Oct. 18, 2001 to Kanao et al. That publication discloses a spark plug center wire assembly with an iridium alloy insert having a stem portion and a head portion attached to a front end of a center electrode. As the head portion of the insert is pressed against the tip of the electrode, a resistance welding operation is applied causing the electrode tip to melt, thereby allowing the head portion of the insert, followed by the stem portion, to sink into the tip. As a result, the melted electrode tip surrounds the head portion of the insert, as well as a portion of the stem, thereby firmly attaching the two components together. The publication also discloses caulking of the electrode tip over the head portion of the insert to secure the insert in place.
Additional methods for attaching precious metal inserts to spark plug electrodes are taught in U.S. Pat. Nos. 3,868,530 and 4,771,210, each of which discloses the use of a mechanical interlock between the two components. In both the '530 and '210 patents, a cylindrical precious metal insert is inserted into the tip of a center electrode, after which an inwardly radial compression is exerted upon the center electrode. This causes deformation of both the center electrode and the precious metal insert such that a mechanical interlock between the two components is formed.
While these and other prior art utilize various methods and techniques for attaching precious metal inserts to center electrodes, including the use of mechanical interlocking features and various types of welds, there still exists room for improvement. For example, hermetically sealed welds between two dissimilar metals can experience stress in the form of expanding gasses trapped between the two materials. This stress can contribute to a failure of the weld such that the precious metal insert may separate from the center electrode.
It is therefore an object of the invention to provide a center electrode assembly which permits secure attachment of the precious metal insert to the center electrode in a manner that allows venting of the internal space between the two components.